114 CRETACEOUS LAMELLIBRANCHIA. 



that whilst a number of living genera of fresh-water mollusks occur in pre-Tertiary 

 rocks, Dreissensia has not yet been found associated with them.^ 



EemarJcs. — Four examples from the Blackdown Greensand were described 

 by Sowerby as distinct species under the names eJentiiUts, lanceulatus, tridens, 

 prseloiigns. Most later authors (d'Orbigny, Forbes, Bronn, Pictet and Renevier, 

 Pictet and Campiche, Briart and Cornet, Stoliczka, Whiteaves, etc.) have considered 

 these forms to be inseparable, and have united them under the name lanceolatiis, 

 but that view was not shared by Morris. An examination of all the available 

 specimens leads me to agree with the opinion generally held. 



In 1850 d'Orbigny regarded the Lower Cretaceous examples as distinct from 

 those fovmd in the Blackdown Grreensand, and named them Mi/tiliis abo-upfus. 

 Pictet and Campiche did not uphold this separation. The only difference that I can 

 detect is that, on the average, the examples found in the earlier beds reach a larger 

 size than those in the later. 



T//pes. — From the Blackdown Greensand : M. cdenfuhis is in the British 

 Museum ; M. tridens and M. pvcBlongus are in the Bristol Museum. I have not 

 been able to trace the type of M. lanceolatus. 



Blstrlhution. — Ferna-he(\, Crackers, and Fitton's Beds 32 and 45, of Atherfield. 

 Ferna-hed of Sandown. Atherfield Beds of East Shalford and Peasmarsh. 

 Ferruginous Sands of Shanklin. Sandgate Beds of Parham Park. Blackdown 

 Greensand (zones x and xv). Greensand of Haldon. Upper Greensand of 

 Shaftesbury. 



i?,„„/^/_MODIOLOPSIDJE, Fischer. 

 Genus — Myoconcha, ./, de G. Soiverbi/, 1824. 



(Min. Couch., vol. v, p. 103, pi. cccclxvii.) 



Myoconcha cretacea, A. d'Orbigui/, 1844. Plate XX, figs. 'S a, b. 



V 1832. Mytilus simplex, A. Pasxy. Gc'ol. de la Seine-iufrr., p. G (expl. of plates), 



pi. xiii, figs. 4, 5. (Non M. simplex, 

 Def ranee, 1824.) 



^ W. J. Sollas, " On the Origin of Freshwater Faunas," ' Scient. Trans. Roy. Dublin Soc.,' 

 ser. 2, vol. iii (1884), p. 106 ; C. A. White, ' Third Ann. Rep. U.S. Geol. Survey ' (1883), p. 423. It 

 has been suggested that Mytilus memhranaceus, Duuker, from the North German Wealden of Obern- 

 kirchen, Egestorf, Oesede, etc., and the Pui'beck beds of Nienstedt and Linden, may belong to the 

 Dreissensiidse, but the characters of the interior of the shell are at j resent vuiknown ; the same may 

 be said of Mytilus Lyelli, Sowerby, from the English Purbeck and Wealden. See Dunker, ' Mon. 

 Norddeutsch. Weald.' (1846), p. 25, pi. xi, f. 10, 11; C. Struckmaun, ' Die Wealdeu-Bildungen von 

 Hannover' (1880), p. 63, pi. i, f. 11, 12; P. Oppenheim, ' Zeitschr. der deutsch. geol. Gesellsch.,' 

 vol. xliii (1891), p. 944. 



